This paper addresses four descriptive research questions. First, is there a direct effect of social background on labour market success over and above the effect of own education? Second, has this effect declined over time? Third, does it vary depending on the level of education achieved and, more precisely, is it weaker among those with higher education. Finally, have the returns on education in the chances of access to the upper class (professional and managerial occupations) varied over time? These questions are addressed for the Spanish case by using comparable social stratification surveys for the years 1988, 1989, 1990-1998, 2005 and 2006. Three measures of success in the labour market (LM) are considered: labour income, a socio-economic index (ISEI) and the chances of access to a given social class. The main findings of the paper are that: there is a considerable direct effect of social origin on LM success; this effect has not changed (and actually if anything it might have increased) over time; and it does not vanish among those with higher education. There is on the other hand evidence of credential inflation, such that the same educational qualification provided better chances to access the most rewarding occupations in the past, compared to nowadays. Once these phenomena and trends are documented, the paper also sets out to investigate possible mechanisms underlying the direct effect of social origins on LM success. It, thus, explores whether the influence of social origins actually become apparent through the choice of field of studies, whether it reflects different abilities captured by performance at school, whether it might hinder social skills related to the family of origin or whether it is due to social networks used in finding a job.